Refactor imgdiag in preparation for ArtMethod

Pull the memory mapping setups and the generic byte-diffing code out of
the primary dump method.  This makes it easier to see the mirror::Object
portions of the code that need to be replicated now that ArtMethod is
no longer in the heap.

Bug: 62554875
Test: adb shell imgdiag --image-diff-pid=<app pid> --boot-image=/system/framework/boot.art
Change-Id: Ib38ebfdd6651a838a9bc5a4100ba561c864746e3